When processing recycling paper, extra substances, such as dirt and printing ink, must be removed from the paper in order to provide a fiber mass usable for further processing. For this purpose, so-called flotation deinking is used rather generally, whereby fiberized and, in most cases, screened fiber mass is fed with air bubbles, which rise and carry impurities upwards in such a way that these can be removed from the surface of the fiber mass suspension. In practice, one aeration is not enough for cleaning the mass, but the known solutions generally utilize several sequential receptacles and different pumping and mixing solutions, trying to expose the suspension to several processes for providing a final result as clean as possible. A problem with such solutions is that the apparatus is complicated and requires separate pumps and tube systems as well as several receptacles. An additional problem with solutions of this kind consists in operating and maintenance costs caused by additional devices.
GB Patent 2 291 367 discloses a solution, in which a suspension is fed into a circular receptacle divided into sectors by means of partition walls and there is a funnel in the middle of the receptacle, into which impurities rising onto the surface of the sectors may flow over the edge of the funnel. The suspension is fed into one sector and it moves forward from the feeding sector into the next sector through openings in the lower portion of the partition walls and finally flows out from the last sector adjacent to the feeding sector. Each sector is fed with air and comprises a separate mixing device, by which the suspension within the sector is put in circulation. In practice, this solution corresponds to several sequential aeration receptacles, even though it functions without separate pumps moving suspension from one aeration space into another. In the solution, however, air feed devices and mixing devices shall be provided for each sequential sector. Finnish Patent Application 922/64 discloses, in turn, a solution, which corresponds to said GB solution and in which suspension is moved from one sector into another by means of so-called air-lift pumps. In other respects, however, the function corresponds to the solution of the GB publication.